A plasma cutting machine has a nozzle disposed to surround an electrode thereof so as to form a fluid passage for an operation gas and throttle a plasma generated in front of the leading portion of the electrode between the electrode and the workpiece to be cut so that the temperature of the plasma is raised and the size of the plasma arc is made to be uniform for the purpose of obtaining an excellent cut surface. However, if the distance between the torch composed of the electrode and the nozzle and the workpiece to be cut is changed, the plasma arc cannot be maintained or an excellent cut shape cannot be obtained because the size of the plasma arc is changed. Hence, the plasma cutting machine has been arranged in such a manner that the distance (hereinafter called a "standoff") between the torch and the workpiece to be cut is kept constant so that the cutting work is performed satisfactorily.
In a case where the standoff is controlled, it has been known that the arc voltage between the workpiece to be cut and the electrode or the nozzle has a proportional relationship with the standoff. The relationship can be utilized to detect the arc voltage, and the detected arc voltage is maintained at a constant value (refer to Japanese Patent Laid-Open No. 57-195582 for example).
However, the aforesaid conventional method is arranged in such a manner that the quantity of the displacement of the torch is simply changed in accordance with the level of the deviation between the detected arc voltage and a reference voltage. Therefore, if the deviation is excessively large, it takes excessive time to realize a predetermined standoff, causing a delay of response to take place. As a result, a satisfactory cutting accuracy cannot be obtained. The aforesaid method is arranged in such a manner that, if the deviation has become excessively large, an abnormal voltage detection circuit disposed individually from a circuit for controlling the standoff detects it so as to turn off the arc voltage. Therefore, the efficiency of the cutting operation deteriorates. What is worse, if the deviation becomes excessively toward negative values, the torch is moved downwardly and the arc is turned off by the abnormal voltage detection circuit so that the deviation is converted into positive values. Hence, the continuation of the downward movement of the torch will cause the torch to collide with the workpiece to be cut, and therefore there is a risk that the torch or the workpiece to be cut is damaged.
Furthermore, consideration has not been taken against the change in the arc voltage with respect to a rise in the arc voltage or a change in the cutting speed due to the consumption of the electrode. As a result, the quantity of the standoff is changed due to the consumption of the electrode or a change in the cutting speed. Therefore, an excellent cutting operation cannot be performed.
In another case in which rebounds of molten metal adhere to a portion adjacent to the nozzle after the torch has approached the workpiece to be cut, or in another case in which the gas flow is disordered due to deformation of the nozzle, a double-arc takes place, for example, between the workpiece to be cut and the nozzle. Hence, the nozzle can be damaged, causing the cutting work to be interrupted. Therefore, an apparatus has been suggested (refer to, for example, Japanese Patent Publication No. 60-43831) which is arranged in such a manner that attention has been paid to the fact that the output voltage from the power source for generating the plasma arc is reduced when the torch comes closer to the workpiece to be cut than a predetermined distance and when the impedance has been reduced, and the output from the power source is shut off if the aforesaid output voltage becomes lower than a reference voltage, so that the torch is protected. However, the cutting operation is temporarily stopped and the working efficiency deteriorates because a command signal, to cause the output from the power source for generating the plasma arc to be zero, is outputted at the time of the generation of the double-arc.
Another method and apparatus has been known (refer to, for example, Japanese Patent Laid-Open No. 62-24864) which detects the use limit of the electrode in accordance with the change of the arc electric current, which is generated between the electrode and the workpiece to be cut, and the voltage. However, since the use limit is detected in accordance with the changes of the waveforms of the electric current and the voltage at the time of the use limit of the electrode, it can be detected after the electrode has been damaged. Therefore, the workpiece to be cut is wasted and the nozzle is damaged. Furthermore, the aforesaid method arranged to detect the use limit in accordance with the voltage cannot easily be put into practical use because the voltage value changes when the cutting conditions are changed.
In order to overcome the aforesaid problems experienced with the conventional technology, an object of the present invention is to provide a standoff control method and an apparatus for a plasma cutting machine capable of quickly correcting deviation of standoff from a set value, capable of keeping constant standoff even if the cutting speed changes, capable of preventing the drop of working efficiency even at the time of occurrence of double-arc and capable of accurately detecting the use limit of an electrode even if the cutting conditions change.